Polyvinyl acetal resin is used in a wide range of applications including wash primers used in metal treatment, various paint compositions, adhesives, resin treating agents, and ceramic binders. Recently, the range of its applications has been expanded to include the electronic material field. In these applications, polyvinyl acetal resin is used in the form of a solution in an organic solvent or an aqueous dispersion.
In light of issues such as environmental pollution, toxicity to human body, and risks in use, more attempts have been made to develop a system mainly made of water which can be used in place of systems containing an organic solvent. This trend has been increasing the importance of polyvinyl acetal aqueous dispersions.
For the production of polyvinyl acetal resin as fine particles, acetalization of polyvinyl alcohol by a precipitation method has been employed. Unfortunately, fine particles of polyvinyl acetal resin prepared by such a method, when dispersed in a dispersion medium mainly made of water, may coagulate into large aggregates, or precipitate within a short period of time upon standing. Once precipitates have formed, they cannot be readily redispersed by stirring or the like. Thus, it is very difficult to maintain the particles dispersed in the medium for a long period of time. For this reason, polyvinyl acetal resin has been typically used as a binder resin dissolved in an organic solvent, and the use of fine particles of polyvinyl acetal resin in the form of an aqueous dispersion has been difficult to achieve.
Aqueous dispersion compositions of a polyvinyl acetal resin that is modified to have improved dispersibility in an aqueous medium have been known as countermeasures to this problem, and RS-3120 (Solutia Inc.) is a typical example thereof.
However, such conventional aqueous dispersion compositions of a polyvinyl acetal resin contain a dispersant, such as potassium oleate or sodium oleate, which makes the dispersions alkaline with a pH of approximately 9. As the pH becomes close to neutral, the dispersing effect of the dispersant decreases, causing the polyvinyl acetal resin to precipitate. In particular, when a compound with high acidity is added to such an aqueous dispersion, the pH around the compound is lowered to cause precipitation even before the pH of the overall aqueous dispersion becomes close to neutral.
As an example of aqueous dispersion compositions of a polyvinyl acetal resin which exhibit stable dispersibility in a neutral pH range, Patent Literature 1 discloses an aqueous dispersion composition of a polyvinyl acetal resin containing at least one dispersant selected from the group consisting of polyoxyethylene tridecyl ethers, polyoxyethylene oleyl ethers, polyoxyethylene lauryl ethers, polyoxyethylene sorbitan monooleates, sodium lauryl sulfate, and potassium lauryl sulfate.
However, the use of these dispersants is also not effective enough to provide sufficient dispersibility in a liquid medium mainly made of water. Another problem is that these dispersants are contained in large amounts in dispersion compositions, and films formed by drying the compositions may have low mechanical properties.
Polymer fine particles have found many applications in the fields of delustrants, anti-block agents, chromatographic carriers, medicine carriers, powder paint compositions, gap adjusters, electrophotographic toners, electrorheological fluids, cosmetics and the like. Polymer fine particles for these applications should have a small particle size and a small particle size distribution, and in particular, in the fields of paint compositions, toners and the like, particles that allow for formation of a film without voids and non-uniform structures such as irregularities are required.
So far, polymer fine particles obtained by a “technique to produce particles by polymerization”, such as emulsion polymerization, suspension polymerization, seed polymerization, or dispersion polymerization, have been used in these applications. Unfortunately, polymer fine particles obtained by emulsion polymerization or suspension polymerization have particle sizes within a limited range and have a broad particle size distribution. Resin particles obtained by seed polymerization or dispersion polymerization are very expensive although they have a sharp particle size distribution.
Polyvinyl acetal resin is a product of acetalization of a polyvinyl alcohol material with an aldehyde, and production of fine particles of polyvinyl acetal resin by the “technique to produce particles by polymerization” has been very difficult.
Although the precipitation method is currently employed to produce fine particles of polyvinyl acetal resin by acetalization of polyvinyl alcohol, fine particles obtained by this method include assemblies of small primary particles, and thus are not spherical fine particles.
Patent Literature 2 discloses spherical porous particles which are obtainable by adding an aqueous solution prepared by mixing an aldehyde and a water-soluble polymer that turns into a gel by ion exchange with polyvinyl alcohol, dropwise to an acid solution, and causing drops to coagulate by the reaction between the polyvinyl alcohol and the aldehyde. Although particles produced by this method are spherical, their porous structure is not suited to the above-mentioned applications. Additionally, the particle size of the particle product is large.